(1) Field of the Invention
The present invention relates to a shape memory alloy, and in particular, to a shape memory alloy having a small temperature difference between the martensitic transition start point and the austenitic transition finish point.
(2) Description of the Prior Art
A typical one of the shape memory alloy is Ti-Ni alloy.
Buehler et al. published in Journal of Applied Physics, 34 (1963), 1467 (reference 1) that Ti-Ni alloy had a unique property which was referred to as, so called, "shape memory effect"(S.M.E.). That is, when cooled, the alloy can easily be deformed below a certain temperature, and thereafter, when heated, the alloy rapidly recovers the original shape above another certain temperature. Thus, the alloy memorizes the original shape.
It is known in the art that the S.M.E. is based on a reverse transition of the martensitic transition which is referred to as the austenitic transition. Cooling the alloy accompanies a phase transition from the austenite to the martensite. The phase transition is called the martensitic transition. The martensitic transition starts from a certain temperature of Ms and finishes at another lower temperature of Mf. Thereafter, when the alloy is heated, the austenitic transition occurs. The austenitic transition starts at a temperature of As and finishes at another temperature of Af. Accordingly, the alloy has a thermal hysteresis in phase transition due to temperature variation.
Ms and Af can be controlled by adjusting an amount ratio of Ni/Ti and also by heat treating the alloy after being cold worked.
Further, the phase transition points of the shape memory alloy, such as the martensitic start point Ms and the austenitic transition finish point Af, shift to the higher temperatures under a stress loaded condition in comparison with no stress loaded condition.
The martensitic transition start point Ms is lower than the austenitic transition finish point Af due to the thermal hysteresis. That is, there is a temperature difference between Ms and Af. The temperature difference will be referred to as a thermal differential hereinafter. The Ti-Ni shape memory alloy usually has the thermal differential of several tens degree in the centigrade. The thermal differential can also be reduced by heat treating the alloy at about 400.degree.-500 .degree. C. after cold working but it is still about 10.degree.-20 .degree. C. which is not sufficiently small.
The Ti-Ni shape memory alloy has recently been used as a thermoresponsive element, for example, a thermoresponsive spring as an actuator. For example, the thermoresponsive spring of the shape memory alloy is expanded by a conventional bias spring and is connected to an object such as a louver to be actuated. When a circumferential temperature elevates above the austenitic transition finish point Af of the shape memory alloy, the thermoresponsive spring shrinks against the stress of the bias spring to recover the original shape and therefore, pulls and opens the louver. Thereafter, when the circumferential temperature lowers below the martensitic transition start point Ms, the thermoresponsive spring is again expanded by the bias spring so that the louver is closed. It is inconvenient that there is a large temperature difference between the louver opening temperature and the louver closing temperature.
Even if the actuator spring is heat treated after cold working, the thermal differential is still large, as described above.
Therefore, it is desirable for thermoresponsive elements that the shape memory alloy has a reduced and small thermal differential.
Further, it is also known that the shape memory alloy has pseudo elasticity or an elasticity based on the stress induced martensitic transition effect. That is, when a stress is applied to the shape memory alloy and is increased at a temperature higher than, but near, the austenitic finish point Af, the stress induced martensitic transition occurs. Thereafter, when the stress is released, the austenitic transition is caused without heating.
Accordingly, although the shape memory alloy is deformed by application of large stress, it recovers the original shape after removal of the stress. Therefore, the shape memory alloy also has some application fields where it is used as a pseudo elastic material. In a certain application field, it is desired that the shape memory alloy has the pseudo elasticity at the room temperature or lower, in particular, about 0 .degree. C.
A shape memory alloy is also known in the art which consists of Ti, Ni, and V, as disclosed in JP-A-53149732 (Tokukai sho 53-149732 which is corresponding to NL-A-7002632) (Reference 2), JP-A-60121247 (Tokukai sho 60-lb 121247 which is corresponding to U.S. patent application Ser. No. 541844) (Reference 3), and a paper entitled "Effect of Additives V, Cr, Mn, Zr on the Transition Temperature of TiNi Compound" by Honma et al in Bulletin of the Research Institute of Mineral Dressing and Metallurgy, Tohoku University, vol. 28, No. 2, Dec. 1972, pp. 209-219 (Reference 4).
Reference 2 discloses that an alloy represented by Ti.sub.1-x NiV.sub.x (0&lt;x.ltoreq.0.21) has a phase transition point between -200 .degree. C. and +20 .degree. C. In particular, a single actual example of Ti.sub.4 Ni.sub.5 V is only disclosed to have the phase transition point between -200 .degree. C. and +20 .degree. C. An amount of V in the example is 10 at %.
Reference 3 discloses a shape memory alloy has the pseudo elasticity or the stress induced martensitic transition effect. In the shape memory alloy, atomic ratio of Ni/Ti is 1.07-1.11, and an amount of V is 5.25-15 at %.
According to our experiment, it is impossible to cold work the alloy containing a high amount of V such as 5 at % or more. It should be noted that samples are worked by casting and machining in references 2 and 3.
Reference 4 only discloses that addition of V into Ti-Ni alloy shifts the martensitic transition start point Ms of the alloy to a lower temperature.
Further, all of the References are silent as to the thermal differential of the alloy.